Evidence accumulated in this laboratory indicates that fibronectin, bound to or part of the platelet membrane is the receptor for collagen and is responsible for the stimulation of the platelet by collagen. Our evidence includes the following observations: 1) A glycoprotein of 230,000 daltons comigrates with fibronectin on SDS slab gels in the presence of a reducing agent, 2) when fibronectin is preincubated with fibrous collagen, the collagen no longer can stimulate platelets, 3) purified anti-fibronectin antibodies stimulate platelets with a dose-related response, while similarly prepared preimmune IgG does not, and 4) platelets contain fibronectin as determined by immune-rocket electrophoresis against anti-fibronectin antibodies. In the forthcoming year the research will include investigations to obtain, if possible, further evidence for the presence and role of platelet fibronectin. In addition, we will investigate the relationship between collagen and the Clq component of complement in relation to fibronectin of platelets.